Piezoelectric transducers are widely used in sensors for the detection of pressures, forces, accelerations, elongations, moments, etc. Thus, piezoelectric pressure sensors are used in pressure indexing of internal combustion engines for detecting a cylinder pressure prevailing in a pressure chamber as a function of a crankshaft position or a time. Internal combustion engines include four-stroke engines and two-stroke engines such as gasoline engines, diesel engines, Wankel engines, etc. In marine diesel engines, they are used for long-term monitoring of a cylinder pressure. However, piezoelectric pressure sensors can also be used in pressure monitoring of jet engines, gas turbines, steam turbines, steam engines, etc.
Frequently piezoelectric crystal material is used for the manufacture of a piezoelectric transducer. The piezoelectric crystal material is cut in a crystallographic orientation to have a high sensitivity for the force to be received. When a force acts onto surfaces of the piezoelectric crystal material, electric polarization charges are generated thereon. The number of the electric polarization charges correlates with the magnitude of the force applied.
The document CH392103A1 demonstrates a piezoelectric pressure sensor having a membrane that is welded with an edge portion to one end of a housing. Membrane and housing serve to protect the piezoelectric transducer from excessive and extreme temperatures during use. The piezoelectric transducer is mounted within the housing behind the membrane. A force received by the membrane acts in a longitudinal direction of the piezoelectric pressure sensor onto the piezoelectric transducer which comprises three bars made of piezoelectric crystal material as well as electrodes in the form of a contact spring and a Bourdon tube. The electrodes are made of electrically conductive material. The contact spring is arranged along a longitudinal axis of the piezoelectric transducer in the center between the bars arranged in an angle of 120° to one another. The Bourdon tube is placed outwardly of the bars with respect to the longitudinal axis. A normal force acting on front faces of the bars generates electrical polarization charges on the side surfaces located transversely to said front faces which electrical polarization charges are transmitted as negative electric charges by the contact spring and as positive electric charges by the Bourdon tube. The Bourdon tube is electrically and mechanically connected to the housing and conducts the positive electric charges to the housing. The contact spring is formed integrally with the charge transmission wherein the charge transmission extends centrally along the longitudinal direction away from the membrane. The charge transmission is electrically and mechanically connected to a socket. The socket is disposed at an end of the housing that faces away from the membrane and accommodates a plug of a line. The socket is electrically insulated from the housing. Thus, negative electrical charges received by the contact spring are fed via the charge transmission to the socket and from the socket to the line. The line itself is electrically and mechanically connected to an evaluation unit where the negative electric charges are amplified and evaluated. Furthermore, the Bourdon spring mechanically pretensions the bars made of piezoelectric crystal material so that tensile and compressive forces can be measured.
In fact, with continuous use the piezoelectric pressure sensor is exposed to strong engine vibrations and high temperatures of 200° C. and above. These may lead to micro friction and fretting corrosion at the contact areas of the side surfaces of the bars being in contact with the contact spring and the Bourdon tube which may lead to weakening of the mechanical stability of the charge transmission. Furthermore, diffusion of base metals and local build-up of oxide layers on the side surfaces of the bars contacting the contact spring and the Bourdon tube may take place at high temperatures. These effects may occur alone or in combination. As a result, the electrical resistance during charge transmission may change. Thus, the electrical contact resistance may increase from the mΩ range by several orders of magnitude into the MΩ range leading to a distortion of charge transmission and to incorrect evaluation in the evaluation unit.
It is a first object of the present invention to suggest a piezoelectric pressure sensor having a charge transmission essentially free from distortion. Another object of the present invention is to provide a pressure sensor wherein the charge transmission is mechanically stable even with strong permanent engine vibrations. Additionally, the manufacture of the pressure sensor shah be cost-effective.